In moving a robot by a playback operation, an acceleration/deceleration control is performed for individual taught points in accordance with a positioning ratio designated in a motion statement in an operation program. Generally, it is advantageous to designate a lower positioning ratio for saving time for acceleration/deceleration to reduce a cycle time. Consequently, it has been a common practice to designate a lower positioning ratio for a taught point which does not require a high degree of positioning accuracy.
However, in a robot for performing an assembling operation by handling a workpiece, for example, if the lower positioning ratio is designated even for the taught point which does not require a high degree of positioning accuracy, there is a problem in operation reliability. FIGS. 1 and 2 show a typical case having such problem.
FIG. 1 shows a moving path in assembling a workpiece 3 by fitting it to another workpiece 4 by gripping the workpiece 3 by a hand 2 attached to a robot arm end 1. FIG. 2 is a chart showing the content of a path motion plan in which a smooth motion of 100% (a positioning ratio of 0%) is designated for the position 2! and a positioning motion of 100% is designated for the position 3!, with an axis of abscissa representing time t and an axis of ordinate representing speed V.
In FIG. 1, 1!, 2! and 3! represent the taught points for a tool point whose position is predetermined with respect to the hand 2 of the robot, and a straight motion is designated for each of the path sections 1!.fwdarw.2! and 2!.fwdarw.3!. Since the position 3! located right under the position 2! is a position at which the fitting operation is completed, a positioning of 100% is necessarily designated in the motion statement. Contrary, since the position 2! is only a transit point where the direction of the movement is changed from horizontal to vertical, it is advantageous to designate "100% smooth", i.e. a positioning ratio of 0% for the position 2! in the motion statement in order to reduce the cycle time.
However, if a lower positioning ratio is designated for the position 2!, an accuracy of the locus would be lowered around the position 2! which forms a corner. Namely, as indicated by reference numeral 5, the robot moves toward the position 3! on a path deflected off the taught path near the position 2!.
Specifically, when "100% smooth" is designated for the position 2! and "100% positioning" is designated for the position 3!, a path motion plan is created so that a motion EFGH (instruction speed V23) for the path 2!.fwdarw.3! in the Z-axis direction starts simultaneously with a start (a point C of time) of a deceleration control movement contained in a motion ABCD (command speed V12) for the path section 1!.fwdarw.2! in the X-axis direction, as shown in FIG. 2. This means that a part (from the point C to the point D) of the motion for the preceding path section 1!.fwdarw.2! has not been completed even at a point (point H) when the motion for the succeeding path section 2!.fwdarw.3! is completed.
As a result, a path approaching the position 3! is deflected from the straight path 2!.fwdarw.3! and there is a possibility that the fitting operation is not performed smoothly. In general, this tendency would be more remarkable when the command speed V12 for the path section 1!.fwdarw.2! is higher and the length L23 of the path section 2!.fwdarw.3! is shorter.
In order to avoid the foregoing problems, it has been a common practice to make a try-and-error adjustment operation including (1) lowering the command speed V12 for the path section 1!.fwdarw.2!; (2) designating a high positioning ratio (for example, 100%) for the position 2!; and (3) locating the position 2! remote from the position 3!.